An efficient interpolation attack

We introduce an efficient interpolation attack which gives the tighter uppe r bound of the complexity and the number of pairs of plaintexts and ciphert exts required for the attack. In the previously known interpolation attack there is a problem in that the required complexity for the attack can be ov erestimated. We solve this problem by first, finding the actual number of c oefficients in the polynomial used in the attack by using a computer algebr a system, and second! by finding the polynomial with fewer coefficients by choosing the plaintexts. We apply this interpolation attack to the block ci pher SNAKE and succeeded in attacking many ciphers in the SNAKE family. Whe n we evaluate the resistance of a block cipher to interpolation attack, it is necessary to apply the interpolation attack described in this paper.